The present invention is directed to a method and to an apparatus for a non-invasive measurement of the velocity of a gas or of a fluid at an investigation location.
The measurement of the flow velocity in gases and fluids at physically inaccessible locations represents an interesting task in broad fields of natural science and technology. The investigations of flow fields around and in technical systems thereby assists in acquiring important information for the optimization of these systems. In the field of investigating the atmosphere, too, the question regarding the flow velocity at physically inaccessible locations is very important. This information is a fundamental importance for the operating safety of the systems in the field of air travel and space travel.
Previously known optical measuring methods can be divided into two classes. The class of Doppler anemometers wherein light is elastically scattered into particles introduced in the medium (gas or fluid) to be investigated given too low a plurality of naturally occurring particles, therefore also referred to as invasive, i.e. no real excitation in the medium occurs, and the class of measuring methods wherein no additional particles are introduced, therefore referred to as non-invasive, wherein an excitation is generated in the medium and the following luminescence is interpreted. The direct interpretation of laser-induced luminescence for measuring velocities in gases is disclosed, for example, by U.S. Pat. Nos. 5,002,389 of T. Benser and 4,483,614 of T. Rogers. In the known methods, a laser pulse is utilized for the excitation of the gas. The typical decay times of the luminescence triggered in this way lie in the range of a few nano seconds through micro seconds. The measuring methods can thus only be conditionally employed for flows having a low velocity.
The invention is thus based on the object of making a method and an apparatus available with which low velocities of a gas over a fluid can also be measured.
This object is inventively achieved by a method for non-invasive measurement of the velocity of a gas or of a fluid at an investigation location, comprising the following steps:
a) Boosting atoms and/or molecules present in the gas or the fluid into an excited atomic or, respectively, molecular state with a first pulsed light beam of a light source focused in the investigation location;
b) Following a time interval xcex94t that is selected of at least a size of the time period for the excited atoms and/or molecules to turn into metastable states due to relaxation, selective boosting of the atoms and/or molecules in the metastable states into an excited atomic or, respectively, molecular state with a second pulse light beam of the light source that is weaker in intensity compared to the first light beam and that transirradiates the investigation location and the suspected, momentary location of the spatial region containing the atoms and/or molecules, so that a characteristic luminescence is produced;
c) detecting the characteristic luminescence of the selectively excited atoms and/or molecules and determining the coordinates of the investigation location and of the occurrence of the luminescence (luminescent cloud) in an image field; and
d) calculating the velocity v of the gas or fluid on the basis of the distance s between the investigation location and the mid-point of the luminescent cloud as well as of the time interval xcex94t as follows:
v=s/xcex94t.
This object is also achieved by an apparatus of the species initially cited in that said apparatus comprises
a light source for generating pulsed light beams;
a first means for focusing a first pulsed light beam of the light source in an investigation location;
a second means for projecting a second pulsed light beam of the light source into a spatial region following a time interval xcex94t;
a third means for detecting luminescence in the spatial region and determining the coordinates of the investigation location and of the occurrence of luminescence (luminescent cloud) in an image field; and
a processing means connected to the third means for calculating the velocity of the gas or of the fluid on the basis of the distance s between the investigation location and the mid-point of the luminescent cloud as well as the time interval xcex94t.
In particular, it can thereby be provided that the direction of the velocity is calculated in addition. The velocity vector is thus obtained.
According to another particular embodiment of the invention, it can be provided that the density of the gas or of the fluid is identified on the basis of the intensity of the characteristic luminescence. The density is thereby proportional to the intensity of the luminescence.
It can also be provided that the characteristic luminescence is additionally temporally detected, and the density of the gas or of the fluid is determined on the basis of the time curve of the characteristic luminescence.
It can be provided according to another particular embodiment of the invention that the characteristic luminescence is additionally spectrally selectively detected and the relative concentrations of the participating atoms and/or molecules are identified on the basis of the relative intensities of the various luminescent spectral lines. Conclusions about the temperature and collisions with other molecules at the location of the excitation can also be drawn on the basis of a more exact analysis of the spectral distribution of the luminescence.
Preferably, the characteristic luminescence is only detected during a fixed time interval xcex94t1 after the selective boost. This leads to an improvement of the signal-to-noise ratio.
According to another particular embodiment of the invention, it can be provided that the velocity of air is identified.
It can thereby be particularly provided that a plasma is briefly generated by the boosting according to Step a).
It can also thereby be provided that N2 molecules are preferably boosted by the plasma generation and these relax into the metastable state A3xcexa3+u after a short time; further, in Step b) the N2 molecules are boosted from the A3xcexa3+u state into the B3Πg state; and, in Step c), the luminescence of the B3Πg state is detected.
It can be provided in the apparatus that the third means comprises a camera.
It can also be provided that the camera is a CCD camera, potentially with image intensifier.
According to a particular embodiment of the apparatus, the third means comprises two PSDs (Position Sensitive Device) that are arranged crossed. In this case, the signal of the two detectors directly corresponds to the transversal coordinates of the luminescent cloud.
Finally, it can also be provided that the third means is spectrally selective.
The invention is based on the surprising perception that the division of the excitation process into two Steps a) and b) that can ensue in a time interval xcex94t that is greater than the typical decay times of the luminescence, velocity measurements can also be undertaken at moving gases or, respectively, fluids. This is possible because the metastable states comprise a life span in the range of milliseconds compared to the preceding luminescence time of 10 through a few 100 nanoseconds. The method can, in particular, be employed for measuring the air velocity in front of a flying aircraft, the turbulent wake around and behind an aircraft, of gas/fluid velocities having high topical resolution without influencing the flow conditions and of flow conditions at elements around which a gas or fluid flows with high topical resolution. Over and above, the inventive method, in a particular embodiment thereof, offers the possibility of additionally defining the density and/or composition at the medium at the investigation location.
Further features and advantages of the invention derive from the claims and from the following specification, wherein an exemplary embodiment is explained in detail on the basis of schematic drawings.